1. Field of the Invention
The present invention relates to the field of mobile telecommunications devices and, in particular, to mobile telephones.
2. Description of Prior Art
A conventional mobile telephone includes a housing comprising a front and rear casing. It is common for the front and/or rear casing to be interchangeable with other casings having a different shape and/or color thereby enabling the owner to customize the appearance of their device as they wish. The front casing is provided with one or more apertures in the vicinity of the earpiece which act as acoustic ducts for the transmission of sound generated by the earpiece through the casing so that it can be heard by the user when the telephone is held against the head.
In many countries, the connection of a mobile telecommunications device to a public network requires that the device be type-approved by a regulatory body. However, for GSM or the “Global System for Mobile Communications”, common specifications have been drawn up so that there is mutual recognition of type approval of all telecommunications devices across Europe and other countries that have adopted the GSM standards. Type approval standards include reference to user safety, compatibility of equipment and the inter-working of mobile telecommunications devices with public telecommunications equipment.
One specific area of the GSM standard defines the acoustic response of the earpiece and specifies type approval limits for the receiving frequency response (RFR) and the receiving loudness rating (RLR).
By changing the shape of the front casing in addition to its thickness and the size/number and distribution of sound transmission holes, the acoustic high frequency response of the earpiece is altered. This causes a problem with type approval because not all exchangeable covers have an identical number/size or distribution of sound transmission holes so it is necessary to ensure that the high frequencies are tuned for every separate front casing that may be used with the device otherwise the type approval limits relating to the acoustic response of the earpiece may not be met. Although this has proved possible with an older generation of mobile telephones having a small range of different cover sets each with a similar number and distribution of sound transmission holes, modern telecommunication devices have a much wider variety of differently designed cover sets each having a completely random distribution and pattern of sound transmission holes. This means that individual high frequency tuning of each cover set to ensure the earpiece has the required acoustic response to meet the type to approval limits set by the GSM standard becomes difficult, if not impossible.
Furthermore, as specific type approval limits must be met and the high frequency response of the telephone is dependent on the shape and configuration of the casing, design freedom is lost as not all desirable cover designs will meet the specified criteria.
In addition to ensuring that any type approval limits are met and the cover is high frequency tuned, it is also necessary to design the earpiece and front cover to ensure that the device is leak tolerance tuned. Leak tolerance has a significant effect on the handling of the telephone. For example, in a device having a non-leak tolerant design, the volume in the human ear will influence the performance an external leak will be created if the telephone is not held tight against the head, thereby significantly altering the sound quality. Therefore, the position of the telephone in relation to the user's ear is a critical factor in the quality of the sound that is heard. On the contrary, with a good leak tolerant system, a natural internal leak is generated from the back of the earpiece to the sound transmission holes in the front casing and the external leaks are minimized so the sound quality is not so heavily dependent on the position of the device relative to the ear. From the foregoing, it will be appreciated that it is desirable to introduce internal loss in the earpiece design and reduce external loss to obtain optimum sound quality that depends very little on the orientation of the telephone with respect to the ear. Conventionally, the casing is provided with sound transmission holes and separate leak holes to provide the required leak tolerance. However, this design is not particularly well suited to exchangeable covers as often, the design allows only very few leak holes.
Finally, the location of the earpiece in relation to the sound transmission holes in the front casing significantly affects the performance of the earpiece. For optimum sound transmission through the front casing, the earpiece should preferably be located centrally and symmetrical relative to the sound transmission holes. If it is non-symmetrical, the sound pressure generated by the earpiece will be reduced substantially. It is also sometimes necessary to position a gasket between the front of the earpiece and the front casing to obtain optimum sound reproduction through the front casing. This also reduces design freedom, as the cover and/or earpiece must be designed to complement each other and the gasket must be a snug fit between the front casing and the earpiece if it is to perform its function correctly.